Farmer, who won three of science fiction's prestigious Hugo Awards, was the prolific author of more than 75 books and scores of short stories during his half-century career.

Philip Jose Farmer was working for a steel and wire company in Peoria, Ill., and writing part time in 1952 when he stirred up the science-fiction world with his first published sci-fi tale, a controversial novella that appeared in the magazine Startling Stories.

"The Lovers," a story in which a male earthling has a sexual relationship with an alien female, broke the taboo against depicting sex in the genre. (The attractive alien, it turned out, was actually a parasitic form of life that mimics humans.)

As Farmer later said in an interview for the online magazine The Zone, "Science fiction never had any sexual relationships in it. I felt that that was a part of life and so should be a part of SF."

Farmer, who won science fiction's prestigious Hugo Award as most promising new talent in 1953 and whose later works included the "World of Tiers," "Riverworld" and "Dayworld" series of novels, died in his sleep at home in Peoria, Ill., on Feb. 25, said his wife, Bette. He was 91.

Once described by writer Harlan Ellison as "a storyteller extraordinaire" and "an intrepid explorer of astonishing places," Farmer was the prolific author of more than 75 books and scores of short stories during his half-century career.

The longtime Peoria resident, whose books have been translated into more than 20 languages and have been published in more than 40 countries, received two other Hugo Awards: in 1968 for best novella, "Riders of the Purple Wage"; and in 1972 for best novel, "To Your Scattered Bodies Go."

In 2001, he was awarded the title Grand Master from the Science Fiction and Fantasy Writers of America for lifetime achievement.

"He is one of the major American science-fiction writers ever," said Gary K. Wolfe, a professor of humanities at Roosevelt University in Chicago and the author of several books on science fiction.

"What he did in terms of influence was to draw on all aspects of literature and put all kinds of different genres together in the same work," Wolfe said. " 'Riders of the Purple Wage' is a very Joycean story full of puns and wordplay, and the same kinds of puns and wordplay might show up in his adventure stories.

"That part of his influence -- just drawing on different genres -- is very much alive today, especially with young writers. At the time he was writing, people either wrote science fiction, or they would write historical fiction or they would write fantasy. He mixed everything up, tore down all the barriers."

Booklist once described Farmer's "Riverworld" saga as "one of the most imaginative worlds in science fiction."

The "Riverworld" novels, Wolfe said, were "set on an artificial planet whose only feature was a million-mile-long river, along the banks of which every human being who had ever lived was mysteriously resurrected. So he could combine any set of historical figures he wanted to. Mark Twain, one of his heroes, is a major character in one of his novels, and Hermann Goering is one of his villains."

Farmer caused another stir in the science-fiction community with "Venus on the Half-Shell," a 1975 novel he wrote under the pseudonym Kilgore Trout, the name of a fictional science-fiction author who appears in a number of novels by Kurt Vonnegut.

Although Farmer later said he received Vonnegut's permission to write the novel, the ensuing speculation over who actually wrote it, including those who fingered Vonnegut as Trout, angered the acclaimed author, who prevented Farmer from writing any sequels.

"It was a sort of tribute to Vonnegut, whose works I admired greatly in the early days," Farmer told Copley News Service in 1999. "It kind of hurt me to have him get teed off about it and in a sense reject me."

Farmer also generated great fan interest in the Wold Newton Family, his literary concept in which all of the 20th century's greatest fictional heroes such as Tarzan, Sherlock Holmes and Doc Savage are actually members of the same family tree.

Born in North Terre Haute, Ind., on Jan. 26, 1918, Farmer moved to Peoria with his family four years later.

Farmer, who began reading science fiction at age 9, spent time in college before joining the Army Air Forces. He was in pre-flight school when the Japanese attacked Pearl Harbor and, according to his website, was discharged after washing out of flight training. He then returned to Peoria and went to work for the Keystone Steel & Wire Co., where he remained for nearly 12 years.

Farmer, who graduated from Bradley University in 1950 with a bachelor's degree in English literature, continued to write fiction while working as a technical writer around the country from the mid-'50s to 1969, when he began writing science fiction full time.

He moved back to Peoria a year later.

In addition to his wife of nearly 68 years, he is survived by his daughter, Kristan Josephsohn; his son, Philip; his sister, Joan; five grandchildren and six great-grandchildren.